revise the tab upload dialog to direct people at the kde store

Summary:
previously the dialog would ask users to mail John Tapsell who I think
doesn't maintain ksysguard anymore, plus that was weirdly not scaling.
instead we can simply publish it through the kde store (D8734).
provide all pertinent information to do this.

also

• switch to xi18n and use markup tags instead of manual rich text
• use KNS to retrieve the category name for the store so as to not hardcode it
• assert that the KNS config has only one category to ensure this doesn't go randomly messed up in the future should multiple categories get used

Test Plan:

• create a new tab
• file -> upload
• dialog pops up
• dialog has relevant information to actually publish the file

Reviewers: Plasma, jriddell, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8735